The authors studied the distribution of alanine aminotransferase (ALT) levels in 10,034 volunteer blood donors. The mean +/- SD ALT value was 21.4 +/- 19.3 IU/liter; 549 (5.5%) of the donors had a ALT value greater that 45 IU; 2.5 per cent had ALT values greater than 60 IU. In general, ALT levels were higher in males than in females, and were age related; peak values occurred in the third decade of life for males and between 50-60 years of age in females. ALT values greater than 45 IU were found significantly more often in males, in donors of both sexes 30-40 years of age, in married donors, in non-Caucasians, and in those whose education level was no higher than high school. Follow-up samples in donors with an initial ALT greater than 45 IU, revealed that 67% continued to have ALT values above 45 IU 2-8 weeks following initial sampling, and 40% had an ALT greater than 45 IU when tested again six months after entry into the study. ALT values greater than 60 IU were associated with a significantly increased prevalence of antibody of hepatitis B surface antigen (anti-HBs) and antibody to hepatitis B core antigen (anti-HBc) occurring together. No statistically significant association was found between transaminase activity and the prevalence of anti-HBs or anti-HBc alone, or with hepatitis A antibody. These findings demonstrate that there are defined sociodemographic and serologic features of donors with elevated ALT values.